An endoluminal prosthesis is a medical device commonly known to be used in the treatment of diseased blood vessels. An endoluminal prosthesis is typically used to repair, replace, or otherwise correct a damaged blood vessel. An artery or vein may be diseased in a variety of different ways. The prosthesis may therefore be used to prevent or treat a wide variety of defects such as stenosis of the vessel, thrombosis, occlusion, or an aneurysm.
One type of endoluminal prosthesis used in the repair of diseases in various body vessels is a stent. A stent is a generally longitudinal tubular device formed of biocompatible material which is useful to open and support various lumens in the body. For example, stents may be used in the vascular system, urogenital tract and bile duct, as well as in a variety of other applications in the body. Endovascular stents have become widely used for the treatment of stenosis, strictures, and aneurysms in various blood vessels. These devices are implanted within the vessel to open and/or reinforce collapsing or partially occluded sections of the vessel.
Stents generally include an open flexible configuration. Furthermore, this configuration allows the stent to be configured in a radially compressed state for intraluminal catheter implantation. Once properly positioned adjacent to the damaged vessel, the stent is radially expanded so as to support and reinforce the vessel. Radial expansion of the stent may be accomplished by inflation of a balloon attached to the catheter or the stent may be of the self-expanding variety which will radially expand once deployed. Tubular shaped structures which have been used as intraluminal vascular stents have included coiled stainless steel springs; helically wound coils which may have undulations or zig-zags therein, slotted stents, ring stents, braided stents and open mesh wire stents, to name a few. Super-elastic materials and metallic shape memory materials have also been used to form stents.
A graft is another type of endoluminal prosthesis which is used to repair and replace various body vessels. Whereas a stent provides structural support to hold a damaged vessel open, a graft provides an artificial lumen through which blood may flow. Grafts are tubular devices which may be formed of a variety of material, including textiles, and non-textile materials. One type of non-textile material particularly suitable for use as an implantable prosthesis is polytetrafluoroethylene (PTFE). PTFE exhibits superior biocompatibility and low thrombogenicity, which makes it particularly useful as vascular graft material in the repair or replacement of blood vessels. In vascular applications, the grafts are manufactured from expanded PTFE (ePTFE) tubes. These tubes have a microporous structures which may allow natural tissue in growth and ceil endothelization once implanted in the vascular system. This contributes to long term healing and patency of the graft.
A stent and a graft may be combined into a stent-graft endoprosthesis to combine the features and advantages of each. For example, tubular coverings have been provided on the inner and/or outer surfaces of stents to form the stent-graft. It is often desirable to use a thin-walled graft or covering in the stent-graft endoprosthesis to minimize the profile of the endoprosthesis and to maximize the flow of blood through the endoprosthesis. In such cases, non-textile materials, such as polymeric tubes or sheets formed into tubes, are often used. Expanded polytetrafluoroethylene or e-PTFE is one common polymeric material used as the graft portion or covering of a stent-graft endoprosthesis. Expanded polytetrafluoroethylene grafts, however, are subject to plastic deformation, especially:1) when compressing the stent-graft for loading into the delivery system, 2) delivering the stent-graft through a highly tortuous bodily lumen and 3) during placement/deployment at the target implant site. Such plastic deformation may lead to the tearing of the ePTFE, leaving the stent-graft endoprosthesis prone to leakage of blood therethrough. Furthermore, plastic deformation of expanded polytetrafluoroethylene grafts may lead to physical deformities in the graft, such as buckling, which is also undesirable because it may lead to poor blood flow patterns.
Sheets or films of ePTFE have been used to cover or line stents. For example, U.S. Pat. Nos. 5,700,285 and 5,735,892 to Myers et al. describe overlapping a sheet of ePTFE onto a stent to form a tubular graft. The graft is secured to the stent by an application of thermoplastic adhesive and heat treatment to melt the adhesive. A seam., which is formed where the sheet overlaps, is also sealed through the use of the thermoplastic adhesive. Such stent-grafts having a unitary tubular ePTFE covering adhesively secured to the stent, however, do not have differential flexibility associated with the graft to augment bending or plastic deformation of the graft when subjected certain stresses, such as bending stresses during delivery through tortuous bodily lumens or loading into a delivery system.
U.S. Pat. No. 6,361,637 to Martin et al. describes the securement or interweaving of ePTFE graft strips through helical windings of an undulating stent wire. The ePTFE strips are spaced apart from the apices of the undulating wire such that no strip completely covers a winding of the undulating wire. The graft strips are secured to the stent wire by use of a thermoplastic adhesive and the application of heat. While such a resulting stent-graft may have additional flexibility as compared to the above-described stent-grafts to Myers et al., the graft wall is non-continuous, thereby not providing by it self a fluid tight graft wall.
Apertures have also been imparted into the walls of tubular ePTFE grafts to increase flexibility. For example, U.S. Pat. No. 6,673,103 to Golds et al. describes a composite stent-graft tubular prosthesis which includes an inner PTFE tubular structure, an outer PTFE tubular structure positioned about the inner PTFE tubular structure and a diametrically deformable stent interposed between the inner and outer PTFE tubular structures. Portions of the stent are exposed exteriorly via apertures in the outer PTFE structure to render the composite prosthesis longitudinally flexible. Moreover, U.S. Pat. No. 6,398,803 to Layne et al. described partially encapsulated stents which are made by using gaps or slits cut into ePTFE covering material. By selecting the size and position, of the apertures or slits in the ePTFE covering, the '803 Patent describes that it is possible to leave certain parts of the stent unencapsulated to facilitate flexibility and expansion of the device. While such stent-grafts may have additional flexibility as compared to the above-described stent-grafts to Myers et al., the graft walls are not continuous, thereby not providing by itself a fluid tight graft wall.
U.S. Pat. No. 6,344,054 to Parodi describes a stent graft having its graft being secured to only one end of the stent. Such a graft avoids undue stresses being placed on the graft during contraction and expansion of the stent by only securing one end of the graft to the stent.
U.S. Patent Application Publication No. 2003/0220682 to Kujawski describes a hybrid braided stent having a plurality of overlapping graft segments. The graft segments are described as being textile graft segments made by, for example, braiding yarns. One end of a graft segment is secured to the stent, and the other end of the graft segment overlaps an adjacent secured graft segment.
Furthermore, ePTFE surfaces have been modified to alter porosity. For example, U.S. Pat. No. 5,466,509 to Kowligi et al. described a more porous ePTFE which is obtained by impressing a pattern, into extruded PTFE and then expanding the PTFE. The pattern is described as being impressed by knurling or rolling a sheet of PTFE sheet between rollers having a pattern formed on the surface of the roller. A roller with a coarse pattern is described as producing a wider distribution of internodal distances of the ePTFE as compared to a finer pattern, thereby increasing the porosity of the ePTFE material.
U.S. Pat. No. 5,462,781 to Zukowski describes an implantable porous expanded polytetrafluoroethylene material having a microstructure of nodes interconnected by fibrils where its surface has been modified by the removal of fibrils so that under magnification the surface has the appearance of freestanding node portions not interconnected by fibrils but rather having open valleys disposed between the freestanding node portions. Unmodified material beneath the surface is described as maintaining the original microstructure of nodes interconnected by fibrils. The modification is described as being done by exposing the surface to radio frequency gas plasma discharge with a reactive etching gas. The modified surface is described as having increased hydrophobicity. Such a modified surface is described as having improved blood contact properties and tissue in growth characteristics useful as an implantable device, such as a breast prosthesis.
U.S. Pat. No. 6,780,497 to Walter describes a process for surface treating ePTFE. The described process employs a laser to surface-modify an ePTFE structure to create a macro-roughened surface that has is described as being highly porous and modified to be highly textured or having increased roughness to promote tissue in growth.
While these later patents describe surface modification or ePTFE to increase porosity or surface modification to increase tissue in growth characteristics, these patents fail to describe modification of ePTFE to improve bending characteristics or to improve flexibility of the ePTFE material as compared to untreated ePTFE,
The endovascular placement of vascular grafts is however subject to several physical performance requirements. One physical performance requirements in particular, is flexibility of the graft or stent/graft combination to permit the device to conform to artery/device angulation and tortuosity. This factor is highlighted especially when the graft or stent/graft combination is implanted in a particular location that requires it to conform to a curved radius of a particular vascular configuration. Typical grafts or stent/graft combinations may not always be flexible enough in the desired location. Stent/graft flexibility is typically a compromise between outward radial expansive force and longitudinal stiffness. Therefore, it is usually desirable that the graft or stent/graft combination have greater flexibility in only a certain defined region. In that way the graft will not compromise its overall structural integrity in order to provide adequate flexibility for certain regions.
For these reasons, it would be desirable to provide an improved prostheses which can overcome some of the drawbacks described above. In particular, it would be desirable to provide intraluminal prostheses which would provide a generally uniform structural integrity, while being able to provide enhanced flexibility in certain defined regions to accommodate and conform to the natural contours of the vasculature in which it is implanted.